1. Field of the Invention
This invention relates generally to methods and apparatus for removing downhole debris such as sand, liquid such as caustic or heavy water, sludge, fish, balls, plugs and the like.
2. Background Art
For a great many years sand lines have been used to remove sand and other debris from well bores. These sand lines usually involve a piston reciprocated within a cylinder through the action of a surface operated wire line connected to the piston. Conventionally the sand lines include a flapper valve at the lower end to prevent reverse flow from the sand line. This type of device is advantageous in comparison to hydrostatic clean out tools in that a large liquid head is not necessary. While devices of this type have been known for some time, present day oil economics have generated renewed interest in what were once considered spent wells and in methods and apparatus for reactivating these wells.
One device of this general type is disclosed in U.S. Pat. No. 4,190,113 to Harrison. The Harrison structure includes a splined piston rod which allows both reciprocatory and rotary motion to be transferred from the surface to the downhole tool. Solid material is collected within a debris retaining chamber while liquid passes through the piston head along the outside of the piston rod, in contact with the cylinder wall, and outwardly through ports for return to the well bore. A removable, gravity operated, flap valve is located at the inlet to the debris retaining chamber. In addition a check valve is located in the piston head in the form of a resilient flap covering a plurality of apertures which extend through the piston head into the region between the piston rod and the cylinder wall.
However, the inventor of the present invention has determined that, in use, structures of the type disclosed in the Harrison patent are subject to a number of shortcomings. Due to the caustic nature of the liquid located in many plugged oil wells, the piston life is sometimes relatively short. This is because the liquid passes around the piston rod and is retained within the piston chamber, damaging the cylinder and piston walls. Since this action is generally not visible to the user and due to the often long operating periods experienced by these devices, their operating efficiency has suffered. This problem is aggravated by the use of nonmetallic valve elements in connection with the piston head that tend to clog and deteriorate in the extraordinary environment within the well which may include not only caustic liquids but also sand, sludge and debris contaminated liquids.
In addition, the structure exemplified by the Harrison patent provides no means for preventing the return of the liquid to the well bore, important, for example, when the liquid debris is caustic, as well as in the cases where it may be desired to withdraw these liquids from the well. For example, the inventor of the present invention has found that caustic or so called "heavy" water is prevalent in plugged oil wells in some regions of western Texas and that it is desireable to remove this water from the well. However, in many other oil fields, often including nearby wells no such problem exists. In wells with uncontaminated water it may be unnecessary to remove the liquid during cleanout.
The structure disclosed by Harrison is also inefficiently prone to dumping its debris load when a gas pocket is encountered within the well bore. Upon exposure to a high pressure gas pocket the contents of the debris retaining chamber are blown upwardly through the pumping chamber and back outwardly through the ported region within the pumping chamber returning the debris to the well bore. In addition to the loss of the time spent collecting the debris in the first place, passage of the debris through the pump tends to foul the pump, making its operation less efficient or sometimes making the pump inoperable. To alleviate, to a degree, these problems it is known to make the piston head removable from the piston rod in the use of the Harrison structure. However, the threaded connection between the rod and piston head is an uncomplete and unreliable solution to these problems.